1. Field of the Invention
The present invention relates to a negative photosensitive polyimide polymer. The polymer has an acid radical, and is developable in an aqueous alkaline solution.
2. Description of the Prior Art
Polyimide (PI) is an organic polymer containing an imide group. As polyimide has excellent thermal stability and good mechanical, electronic, and chemical properties, it has always been the most popular high-performance polymer. In recent years, as requirements on properties have become more and more stringent in the semiconductor industry, and there are limits and defects in the application of the traditional inorganic materials, the use of polyimide can make up for the deficiencies of the traditional inorganic materials in some ways. In the semiconductor industry with stringent requirements on materials, polyimide is also used in passivation layers, stress buffer coatings, a particle masking films, dry etching protection masks, interlayer insulating films, and microelectromechanical applications and the like, while other new uses are under further development. Currently, among these applications, polyimide is primarily used in the coatings for protecting an integrated circuit (IC) element, mainly because the polyimide materials can pass the reliability test of the IC element. In fact, in addition to the application in IC industry, polyimide provides other important applications in electronic packaging, enameled wire, printed circuit board, sensing element, separating film and construction material, as a critical material.
Polyimides utilized in IC processes may be classified as the conventional non-photosensitive polyimides and the photosensitive polyimides (PSPI). The conventional polyimides have to be used in combination with a photoresist to provide the desired circuit pattern. In case of the photosensitive polyimides, a photosensitive group is present the main structure of the polyimides so as to impart the polyimides with photosensitivity, which allows the polyimides to have the properties associated with an insulating layer and photoresist, and can provide the desired circuit patterns upon direct exposure and development. In a semiconductor process, the application of photosensitive polyimides can save the amount of photoresist materials used, reduce the process steps and the cost, and improve the yield. Therefore, most of the semiconductor manufacturers have changed to use photosensitive polyimides.
In common lithography processes, if positive working photoresists are used, sidewall undercutting tends to occur upon development, resulting in the structural size error, which is even more severe when exposure dose is excessive. Accordingly, current commercially available photosensitive polyimides primarily are negative type systems, and can be classified into three kinds of polymers according to the molecular structures thereof:                1. Covalently Bonding Polyamic Esters (PAEs):        

This system was developed by Siemens Corp., Germany, wherein the photosensitive crosslinking group, such as hydroxyethyl methacrylate (HEMA), was covalently bonded to the dianhydride via a nucleophilic reaction, then polymerized with the diamine monomer to produce the polyamic ester. In view of the difference in molecular structure, the covalently bonding photosensitive resin can create a negative type pattern by being exposed to a light source, where the exposed portion is crosslinked and will not dissolve in organic solvents, while the unexposed portion is soluble in organic solvents.                2. Ionically Bonding Polyamic Acids:        
Based on a similar principle as that associated with the covalently bonding photosensitive polyimide, Toray Corp., Japan, mixed a polymerized polyamic acid with a tertiary amine having a photosensitive crosslinking group, so that the photosensitive crosslinking group was bonded to the backbone of the polyamic acid to produce a photosensitive material.
                3. Self-photosensitive Polyimides:        
This system was developed by Ciba-Geigy Corp., Switzerland, which is a soluble, completely imidized polyimide obtained by the reaction of benzophenonetetracarboxylic dianhydride (BTDA) with the diamine having an alkyl at the ortho-position. Such a material is self-photosensitive without a photoinitiator. Upon photoirradiation, the benzophenone group in the excited state captured the hydrogen atom from the alkyl at the ortho-position, causing crosslinking reaction and rendering the resin non-soluble.
Among the above-mentioned three systems, the covalently bonding photosensitive polyimides have better performance in lithographic imaging, and particularly are able to produce a photosensitive coating having a film thickness up to 100 μm, but is more costly. Although the photosensitive polyimides made by ionically bonding is less costly and easy to be synthesized, it is poor in performance as a result of insufficient strength in ionic bonding, and even the exposed crosslinked portion could be washed off by the developer. As for the self-photosensitive polyimides, they are seldom used due to the low photosensibility.
Moreover, the formula of the final product for a common photosensitive polyimide is somewhat complicated, that is, in addition to the polymer itself, a photoinitiator, a crosslinker, and a promoter should be added to improve the photosensitivity, resolution, and adhesion ability. In addition, most of the photosensitive polyimides need to be developed with an organic solvent, such as xylene, cyclohexanone, cyclopentanone or γ-butyrolactone. However, the organic solvent tends to cause swelling, which is detrimental to the resolution, and results in severe environmental pollution, and more wastewater treatment equipments, and therefore cost, are necessary.
In view of the shortcomings of the above-mentioned photosensitive polyimides in application, the present inventor provided, through extensive studies, a novel negative photosensitive polyimide that is developable in an aqueous alkaline solution and provides thermal stability, high resolution and high adhesion, thus being more satisfactory in industry.